Precession is the gradual, slow wobble or change in the orientation of the axis of a rotating object, like the Earth. In the case of Earth, this refers to the precession of the equinoxes, which is caused by the gravitational forces exerted by the Sun and the Moon on Earth’s equatorial bulge. Over time, this causes the Earth’s axis to slowly shift its direction in space, although the tilt of the axis remains roughly the same.
How Precession Affects the Celestial Sphere:
The celestial sphere is an imaginary sphere that surrounds Earth, used to map the positions of celestial objects. Precession has the following impacts on it:
- Change in the North Star: Because of precession, the Earth’s axis shifts over time, so the star that is closest to the North Pole (which we currently know as Polaris) changes. In about 12,000 years, Vega will be the North Star, and in another 26,000 years, Polaris will once again be the North Star.
- Shift in the Equinoxes: Precession causes the points where the celestial equator intersects the ecliptic (the celestial equinoxes) to slowly shift. This means the position of the equinoxes moves gradually along the ecliptic, completing a full cycle approximately every 26,000 years (this is also called the “Great Year”).
- Zodiacal Constellations: The precession of the equinoxes affects the apparent position of the Sun against the backdrop of the zodiacal constellations. The constellations that the Sun appears to pass through over the course of a year slowly change due to this precession, which is why the traditional astrological signs don’t align with the positions of the stars today as they did thousands of years ago.